The present invention relates generally to a method of fabricating a multilayer printed-circuit board having a plurality of internal layers and in particular to a method of assuring stability of the internal layers in the laminate fabrication.
In view of the tendency of electronic devices being produced with a continuously increasing integration density, there arises a great demand for the realization of a multilayer printed-circuit board with a higher density for mounting of such electronic devices. To meet the demand, attempts have been made to increase the number of layers of the multilayer printed-circuit board. Increasing the number of the internal layers however involves a corresponding increasing in the overall thickness of the multilayer printed-circuit board. On the other hand, in response to the demand for the high density of interconnections among conductor patterns, approaches have been made to decrease the diameter of through-holes formed in the board, bringing about various problems such as breakage of drilling tool or bit in drilling small-diameter holes of an increased length corresponding to the increased overall thickness of the board, defects in the plating of the through-holes and poor reliability thereof. Accordingly, it is desirable to suppress an increase in the overall thickness of the multilayer printed-circuit board to as great a degree as possible. To this end, the thickness of the individual internal layer sheet as well as that of prepreg used as an adhesive sheet should desirably be as thin as possible. However, a decrease in the thickness of internal layer sheets and prepreg sheets will result in an increased deviation of dimensional changes of a specific internal layer sheet on a board-by-board basis. With the phrase "dimension of the internal layer sheet", it is intended to mean the dimension measured along the plane of the internal layer. The internal layer sheets stacked one another with prepregs interposed undergo changes in dimension in the course of the multilayering or laminating process under the influence of heat and pressure treatments. Here, with the phrase "deviation of the dimensional change", it is intended to mean the non-uniformity or dispersion of changes in dimension between predetermined layer surface locations of a specific internal layer sheet among the difference multilayer printed-circuit boards, particularly those locations of an internal layer sheet having the signal layers which are imposed with strict dimensional tolerance requirement.
Table 1 shows, by way of example, the results of experiments conducted for determining the deviation of the dimensional changes among the internal layer sheets prior and subsequent to the laminate process by changing the thickness of the prepreg and the insulation portion of the internal layer sheet which is composed of copper-plated epoxy-glass.
TABLE 1 ______________________________________ Experiment I Experiment II ______________________________________ Thickness of insulation 0.2 0.1 portion of internal layer sheet (mm) Thickness of prepreg (mm) 0.1 0.05 Deviation of dimensional 100 230 change among internal layer sheets* ______________________________________ *Note: Deviation measured in experiment I is standardized to 100
As will be seen in the Table 1, when the thickness of the insulation portion of the internal layer sheet and that of the prepreg are reduced by a half, the deviation of the dimensional change among the internal layer sheets increases by twice or more. For the reason described above, the thickness of the insulation sheet of 0.2 mm and that of the prepreg of 0.1 mm have presented practical limits in the hitherto known multilayer printed-circuit board. As the consequence, the overall thickness of the multilayer printed-circuit board has been necessarily increased in accompaniment to the increase in the number of layers. By the way, concerning the dimensional changes of the multilayer composite which occur in the lamination process is reported in an article entitled "A New Set of Printed-Circuit Technologies for the IBM 3081 Processor Unit" of IBM J. RES. DEVELOP, Vol. 26, No. 1, January, 1982.